bug#43865: mml-attach-file could call system file chooser

[Eli Zaretskii]

> From: 積丹尼 Dan Jacobson
>  <jidanni@jidanni.org>
> Date: Thu, 08 Oct 2020 19:45:59 +0800
> 
> C-c C-a runs the command mml-attach-file (found in message-mode-map),
> which is an interactive autoloaded compiled Lisp function in
> ‘mml.el’.
> 
> OK, but wouldn't it be nice if this could call the "system file chooser"
> like Firefox and Chromium do when picking files?

Emacs pops up the file selection dialog when the command that prompts
for a file was invoked from a mouse-click event (some menu or tool-bar
button).  When the command is invoked from the keyboard, we don't show
the GUI dialogs.  See the variable use-dialog-box.

This is not a bug, but the usual Emacs behavior.